a) Field of the Invention
This invention relates to a camera, and more particularly to a single-lens reflex camera.
b) Description of the Related Art
In the past, what is called a TTL system which is free of parallax has been adopted as a focus detecting apparatus for single-lens reflex camera.
An example of the conventional focus detecting apparatus of this type is schematically shown in FIG. 1 with reference to the optical system of the apparatus which is incorporated in the single-lens reflex camera and commercially available. This optical system comprises a photographic lens 1; a swing-up type instant-return mirror 3 composed of a half mirror, pivotally mounted in the mirror box 2; a sub-mirror 4 pivotally mounted at a point Q on the back surface of the instant-return mirror 3, located perpendicular to the instant-return mirror 3 when the instant-return mirror 3 is set at its lower position (shown) and coming in close contact with the back surface of the instant-return mirror 3 when the instant-return mirror 3 is shifted to the upper position; a film plane 5 arranged, behind the sub-mirror 4, on the rear face of the mirror box 2; a field stop 6 arranged, on the optical path of reflection from the sub-mirror 4, at a plane equivalent optically to the film plane 5; a condenser lens 7 arranged adjacent to the field stop 6; a mirror 8 arranged behind the condenser lens 7; an separator stop 9 arranged behind the mirror 8, having a pair of apertures juxtaposed normal to the plane of the figure at an interval which makes it possible to secure focusing accuracy; a pair of separator lenses (reimaging lenses) 10 juxtaposed, normal to the plane of the figure, behind the pair of apertures of the separator stop 9, respectively; a light-receiving element 11 arranged at the imaging position brought about by the separator lenses 10; a focusing screen 12 arranged, on the optical path of reflection from the instant-return mirror 3, at a plane equivalent optically to the film plane 5; a condenser lens 13 disposed in front of the focusing screen 12; a field stop 14 disposed at the same position as the focusing screen 12; a pentagonal prism 15 whose entrance surface is positioned in rear of the focusing screen 12; and an eyepiece 16 arranged behind the exit surface of the pentagonal prism 15. Of these components, the photographic lens 1 and film plane 5 constitute a photographing optical system. Further, the instant-return mirror 3, sub-mirror 4, field stop 6, condenser lens 7, mirror 8, separator stop 9 with a pair of apertures, separator lenses 10 in a pair, and light-receiving element 11 constitute a focus detecting optical system on a phase correlation scheme of performing the focus detection by making use of beams of light traversing the different areas of the photographic lens 1 to detect the relative positional relationship of a pair of images of an object. Also, the instant-return mirror 3, condenser lens 13, focusing screen 12, field stop 14, pentagonal prism 15, and eye-piece 16 constitute a finder optical system.
Specifically, when the instant-return mirror 3 is set at its upper position, a primary image I.sub.0 of an object point 0 is formed on the film plane 5 by the photographic lens 1. On the other hand, when the instant-return mirror 3 is set at its lower position, part of the imaging beams of light through the photographic lens 1 is transmitted by the instant-return mirror 3 and conducted to the focus detecting optical system and the remainder, reflected from the instant-return mirror 3, is conducted to the finder optical system.
For the focus detecting optical system, the light beam transmitted through the instant-return mirror 3 is reflected at the sub-mirror 4 so that a primary image I.sub.1 of the object point 0 is formed on the field stop 6 and then, through the condenser lens 7, mirror 8, separator stop 9 with a pair of apertures, and separator lens 10 in a pair, a pair of secondary images I.sub.2 and I.sub.3 is formed on the light-receiving element 11.
As for the finder optical system, the light beam reflected from the instant-return mirror 3 is nearly collected by the condenser lens 13 so that a primary image I.sub.1 ', after being formed on the focusing screen 12, is converted into an orthographic image, together with the image of the field stop 14, by the pentagonal prism 15, the orthographic image being magnified through the eyepiece 16 for observation at an eyepoint EP.
In the focus detecting optical system, the image of an exit pupil P at the photographic lens 1 is formed on the separator stop 9 by virtue of the condenser lens 7. By the pair of apertures of the separator stop 9 and the pair of separator lenses 10, two beams of light traversing the different areas of the exit pupil P are conducted onto the light-receiving element 11 while holding a certain base length (the distance between the light beams passing through the centers of the apertures of the separator stop 9).
Also, there is a demand for the focus detecting apparatus in which the focus detection can be performed with respect to the periphery of a picture plane to be photographed as well as the center thereof. Such focus detecting apparatus are set forth, for example, in Japanese Patent Preliminary Publication No. Sho 63-131111 and Japanese Patent Preliminary Publication No. Hei 4-6509 proposed by the same applicant as in this application. The former is adapted to comprise discretely a plurality of focus detector assemblies in the direction of a major side perpendicular to the optical axis (principal optical axis) of the photographic lens 1. The latter, on the other hand, is such that focusing can be made at any position of the focus detector assemblies extending in the directions of major and minor sides of the picture plane to be photographed. FIG. 2 is a diagram showing an essential part of the optical system of the latter. This figure shows only an imaging optical system subsequent to the field stop of the focus detecting optical system. The imaging optical system includes a field stop 17 in which a wide field of view is set, having an aperture 17a (for example, of a rectangle); a condenser lens 18 performing principally the transfer of a pupil image; an aperture stop 19 having a pair of apertures 19a and 19b deviated, by .delta., from the principal optical axis of the condenser lens 18; and a first separator lens 20 and a second separator lens 21 which have positive refracting powers. Of the entrance surfaces of these two separator lenses 20 and 21, at least one is configured as an aspherical surface. In such an instance, the condenser lens may be a two-lens unit consisting of lenses 22 and 23 as depicted in FIG. 3 and each of the separator lenses 20 and 21 may be separately constructed on both sides of the principal optical axis as shown in FIG. 4. By this arrangement, the light beams passing through the different areas of the exit pupil P of the photographic lens 1 are formed as the primary image with the wide field of view at the field stop 17 so that the primary image, after being somewhat magnified by the condenser lens 18, passes through the pair of the apertures 19a and 19b of the aperture stop 19, is relayed by the first and second separator lenses 20 and 21 and formed as two secondary images with parallax which are inverted real images, on the sensor surface of the light-receiving element 11. Such an arrangement makes it possible to perform focusing at any position of the focus detector assemblies extending in the directions of the major and minor sides of the picture plane, but a primary imaging plane needs to extend in the directions of the major and minor sides as a matter of course. As shown in FIG. 1, however, in the case where the optical system includes the swing-up type instant-return mirror 3 composed of a half mirror, pivotally mounted in the mirror box 2 and the sub-mirror 4 pivotally mounted on the back surface of the instant-return mirror 3, located perpendicular to the instant-return mirror 3 when the instant-return mirror 3 is set at the lower position and coming in close contact with the back surface of the instant-return mirror 3 when the instant-return mirror 3 is in the upper position, the sub-mirror 4 must be enlarged to expand the primary imaging plane, so that the mirror box 2 becomes imperatively large and the camera body also increases in size, with unfavorable results. Moreover, it is required that the back focus of the photographic lens is increased, which imposes great restrictions on the design of the photographic lens, also with unfavorable results.
To prevent the mirror box 2 from being enlarged, Hei 2-108195 proposes an arrangement such as shown in FIG. 5 by way of example. Specifically, a condenser lens 24 and a half mirror 25 are arranged on the optical path of reflection from the instant-return mirror 3 constructed of an ordinary reflecting member, and the aperture stop 19 with a pair of apertures, a pair of separator lens units 26 composed of the first and second separator lenses 20 and 21, and the light-receiving element 11 are arranged, in the order described on the optical path of reflection from the half mirror 25 to constitute the focus detecting optical system. Further, a mirror 27 is arranged on the optical path of transmission through the half mirror 25, and an aperture stop 28, a field transfer lens 29, a pupil transfer lens 30, and the eyepiece 16 are arranged on the optical path of reflection from the mirror 27 to constitute the finder optical system.
For the field image transfer, in the finder optical system, the field image I.sub.1 formed on the condenser lens 24 by the photographic lens 1 is further formed adjacent to the pupil transfer lens 30 as the image I.sub.2 erected by the half mirror 25, mirror 27, and field transfer lens 29, and the image I.sub.2 is magnified and observed as an orthographic virtual image through the eyepiece 16. In the finder optical system, on the other hand, the field image I.sub.1 is formed, as two images I.sub.3 and I.sub.4 with paralax, on the light-receiving element 11 by the half mirror 25 and the separator lens units 26. In such a case, to form the two images I.sub.3 and I.sub.4 with parallax, a diffuser, a split image prism, or a microprism cannot be disposed in the area of the field image I.sub.1. Focusing through the eye can be made by providing the diffuser, split image prism, or microprism in the area of the image I.sub.2 erected by the field transfer lens 29.
As for the pupil image transfer, in the finder optical system, the image of the exit pupil P at the photographic lens 1 is transferred through the instant-return mirror 3, condenser lens 24, half mirror 25, and mirror 27 onto the aperture stop 28 and then, through the field transfer lens 29, pupil transfer lens 30, and eyepiece 16 to the eyepoint EP. In the focus detecting optical system, on the other hand, the image of the exit pupil P is transferred through the instant-return mirror 3, condenser lens 24, and half mirror 25 onto the aperture stop 19.
According to the foregoing arrangement, the half mirror 25 splitting the light beam directed toward the finder optical system in the direction of the focus detecting optical system is arranged in rear of the instant-return mirror 3 directing the light beam from the photographic lens 1 toward either the film plane 5 or the finder optical system, so that the mirror box 2 dispenses with the need for the sub-mirror 4 pivotally mounted on the back surface of the instant-return mirror 3 and can be constructed without any increase in size.
The use of such an arrangement certainly makes it possible to perform focusing at any position of the focus detector assemblies extending in the directions of the major and minor sides of the picture plane and prevents the mirror box 2 from increasing in size. This arrangement, however, has encountered the problems that the optical path length cannot be secured for using the pentagonal prism in the finder optical system like the conventional manner and the volume occupied by the finder and focus detecting optical systems increases, with the result that the entire camera body becomes bulky.